The tank NPC follows a specific pattern when pulling: they run into range to use their ranged skill, then immediately use their AoE skill once, and then start their single-target rotation. What I've found works best for ensuring that the tank gets aggro on groups is to Engage them on the furthest target away. That way they should be in the middle of the enemy group when they use their AoE skill. Of course, if your tank NPC is a marauder they still likely won't hit everything. The tanks will eventually use Ultimatum, too, but I've seen no real rhyme or reason behind when it triggers aside from "it takes a while".